It is known that a three-way catalyst, a storage reduction NOx catalyst (NOx storage reduction catalyst) (hereinafter referred to as “NSR catalyst” as well), and a selective catalytic reduction NOx catalyst (hereinafter referred to as “SCR catalyst” as well) are arranged in this order from the upstream side of an exhaust gas passage of an internal combustion engine which can be operated at a lean air-fuel ratio. The NSR catalyst occludes (absorbs or stores) NOx contained in the exhaust gas when the oxygen concentration of the inflowing exhaust gas is high, while the NSR catalyst reduces NOx having been occluded when the oxygen concentration of the inflowing exhaust gas is lowered and any reducing agent is present. The SCR catalyst selectively reduces NOx by means of the reducing agent. Then, HC and/or H2 contained in the exhaust gas is/are reacted with NOx in the three-way catalyst or the NSR catalyst, and thus NH3 is produced. This NH3 can be utilized as the reducing agent in the SCR catalyst.
In the construction described above, the rich spike, by which the air-fuel ratio of the exhaust gas is temporarily made to be a rich air-fuel ratio, is executed in order to produce NH3 in the three-way catalyst or the NSR catalyst. In this context, a technique is known, in which the target air-fuel ratio is switched during the rich spike from a first air-fuel ratio to a second air-fuel ratio which is the air-fuel ratio higher than the first air-fuel ratio (see, for example, Patent Literature 1). In this way, NH3 can be produced at an early stage by promptly releasing oxygen from the three-way catalyst and the NSR catalyst by further lowering the air-fuel ratio of the exhaust gas at the initial stage of the rich spike.
In the meantime, even in the case of the internal combustion engine which can be operated at the lean air-fuel ratio, the internal combustion engine is also operated at the theoretical air-fuel ratio, for example, during the high load operation in some cases. When the internal combustion engine is operated at the high load, then the temperature of the SCR catalyst is raised, and NH3, which has been adsorbed by the SCR catalyst, is sometimes released. Further, if the operation period at the theoretical air-fuel ratio is prolonged, then NH3 cannot be produced by the three-way catalyst and the NSR catalyst, and hence it is impossible to supply NH3 to the SCR catalyst. In this situation, when the air-fuel ratio is switched from the theoretical air-fuel ratio to the lean air-fuel ratio, it may be difficult to purify NOx due to the shortage of the reducing agent in the SCR catalyst. In this context, when the operation is performed at the theoretical air-fuel ratio, NOx is released from the NSR catalyst. Therefore, NOx can be occluded in the NSR catalyst after the operation is switched from the operation at the theoretical air-fuel ratio to the operation at the lean air-fuel ratio. However, it is feared that a part of NOx may flow out from the NSR catalyst without being occluded by the NSR catalyst depending on the operation state of the internal combustion engine. In such a situation, if NOx cannot be purified by the SCR catalyst, it is feared that the NOx purification rate of the entire system may be lowered. Therefore, when the operation is transferred to the operation at the lean air-fuel ratio after the operation is performed at the theoretical air-fuel ratio, it is desirable to promptly supply NH3 to the SCR catalyst.
On the contrary, it is conceived that NH3 is produced by the NSR catalyst when the operation is switched from the operation at the theoretical air-fuel ratio to the operation at the lean air-fuel ratio. That is, it is conceived that the rich spike is carried out in order to produce NH3 by the NSR catalyst. However, NOx is released from the NSR catalyst when the operation is performed at the theoretical air-fuel ratio. Therefore, the NOx occlusion amount of the NSR catalyst is decreased when the air-fuel ratio is switched to the lean air-fuel ratio, and it becomes difficult to produce NH3 by the NSR catalyst in some cases. On the other hand, even when NH3 is produced by the three-way catalyst, if oxygen exists in the NSR catalyst, then it is feared that NH3, which is produced by the three-way catalyst, is consequently reacted with oxygen in the NSR catalyst, and NH3 does not arrive at the SCR catalyst. Therefore, it is feared that NH3 cannot be supplied to the SCR catalyst immediately.